Process for changing material in injection molding and sandwich-shaped molded article

ABSTRACT

When a first molten plastic plasticized by a first injection unit  41  is changed to a third molten plastic by using a sandwich molding nozzle capable of using first and second molten resins, which are plasticized by first and second injection units  41  and  42 , for any one of skin and core layers, a mixed resin of the first and third molten resins supplied from the first injection unit  41  is fed to the sandwich molding nozzle so as to be used for the core layer to carry out a sandwich molding process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a process for changing material in injection molding and a sandwich-shaped molded article. The invention more particularly concerns with a process for changing material in injection molding, capable of reducing the quantity of a plastic waste from an injection molding machine during a plastic material changing process (including a color changing process).

[0003] 2. Description of Related Art

[0004] It is conventionally required for a manufacturing processes of a molded articles to change a plastic material to a different plastic material, which are injected from the injection molding machine. In order to completely replace a previous plastic material in an injection unit with a next plastic material, there is known a method of continuing a injecting process using a mixed molten plastic material while the previous plastic material is force out by the next plastic material. There is another conventional material changing method. In this case, an injection molding process is interrupted to repeat charging and purging previous material until the previous material in the injection unit is replaced with the next material completely.

[0005] Both of such material exchanging processes are wasteful of large quantity of plastic material, increasing the costs of the sandwich-shape molded articles. In addition, the large quantity of plastic wastes is harmful to the environment. To prevent any damages to the environment, the costs of disposing the wastes add up considerably.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a resin changing process for injection molding, the method being capable of effectively using a mixed plastic material, which exists during a resin changing process, for molded articles to reduce the quantity of useless material, and a sandwich-shaped molded article which is produced by using such a method.

[0007] In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a material changing process for an injection molding, said method comprising the steps of; providing a first injection unit for plasticizing and injecting a first molten material and a second injection unit for plasticizing and injecting a second molten material and an injection nozzle connected to the first and second injection unit so that either of the first molten material or the second molten material selectively is injected into a mold ;changing the first molten material being plasticized in the first injection unit to a third molten material; injecting the second molten material in the second injection unit into the mold to form a skin layer of a sandwich-shaped article; injecting a mixed molten material being mixed the first molten material with the third molten material in the first injection unit into the mold to form a core layer of the sandwich-shaped article; and repeating a injection molding cycle for molding the sandwich-shaped article until the mixed molten material in the first injection unit is completely replaced with the third molten material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention. However, the drawings are not intended to imply limitation of the invention to a specific embodiment, but are for explanation and understanding only.

[0009] In the drawings:

[0010]FIG. 1 is a schematic diagram showing an example of a injection nozzle for use in a material changing process for injection molding according to the present invention;

[0011]FIG. 2 is a schematic diagram showing an example of a injection molding process for a sandwich-shaped article wherein (a) shows a state that a molten plastic material for a skin layer is initially injected into a mold, (b) shows a state that for a core layer is then injected into the mold, and (c) shows a state that a molten plastic material for a skin layer is finally injected into the mold; and

[0012]FIG. 3 is a schematic diagram showing another example of injection molding process for the sandwich-shaped article.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0013] Referring now to FIGS. 1 through 3, the preferred embodiments of the present invention will be described below.

[0014]FIG. 1 shows an example of a injection nozzle for injection molding a sandwich-shaped article which is applicable to material changing method according to the present invention.

[0015] In FIG. 1, reference number 10 denotes a nozzle body, 11 denoting an outer nozzle, 12 denoting an inner nozzle, 13 denoting an outlet, 15 denoting a first channel, 16 denoting a second channel, 17 denoting a first port, 18 denoting a second port, 31 denoting a first supply passage, 32 denoting a second supply passage, 33 denoting a first shut-off valve, and 34 denoting a second shut-off valve. The terms “first” and “second” are used only for distinguishing one from the other, and one may be first and the other may be second.

[0016] The injection nozzle for molding the sandwich-shaped article comprises the nozzle body 10 and the connecting block 30. A first injection unit 41 and a second injection unit 42 which are sources of molten plastic material are connected together by the connecting block 30.

[0017] The nozzle body 10 comprises the outer nozzle 11 and the inner nozzle 12 disposed coaxially therein. The outer nozzle 11 has the outlet 13 at the tip end of the outer nozzle 11. As shown in FIGS. 2 and 3, the tip end of the outer nozzle 11 is detachably connected to a gate 21 which is formed in the back side of a mold 20. The rear end of the outer nozzle 11 is fixedly secured to the connecting block 30.

[0018] The first channel 15 extends along the axis of the inner nozzle 12 to the outlet 13 of the outer nozzle 11. The second channel 16 in the shape of a annulus is defined between the outer peripheral surface of the inner nozzle 12 and the inner peripheral surface of the outer nozzle 11. The first channel 15 is connected to the second channel 16 are configured to join in the downstream end of the inner nozzle 12 to be communicated with the outlet 13.

[0019] The first port 17 for introducing a first molten plastic material discharged from the first injection unit 41 into the nozzle body 10 is formed at the upstream end of the inner nozzle 12. The first port 17 is opened into the first channel 15 which is communicated with the outlet 13. The second port 18 for introducing a second molten plastic material discharged from the second injection unit 42 into the nozzle body 10 is formed at upstream end of the outer nozzle 11. The second port 18 is opened into the second channel 16 which is communicated with the outlet 13.

[0020] The connecting block 30 has a first supply passage 31 and a second supply passage 32. The upstream end of the first supply passage 31 is connected to the first injection unit 41. The downstream end of the first supply passage 31 is connected to the first port 17. A first shut-off valve 33 of a rotary type is provided in the first supply passage 31, midway down from the first injection unit 41. Similarly, the upstream end of the second supply passage 32 is connected to the second injection unit 42. The downstream end of the second supply passage 32 is connected to the second port 18. A second shut-off valve 34 of a rotary type is provided in the second supply passage 32, midway down from the second injection unit 41.

[0021] The first shut-off valve 33 and the second shut-off valve 34 perform a function for changing selectively the molten plastic marerial to be injected into the mold 20. Therefore, the injection nozzle can enable the first and second injection units 41,42 to carry out not only a injection molding by operating the first or second injection units 41,42 selectively and independently but also a injection molding for a sandwich-shaped article by operating a combination of the first and second injection units 41,42.

[0022] As shown in FIG. 2, the sandwich-shaped article comprises a skin layer and a core layer. The first molten plastic material delivered from the first injection unit 41 is formed into the skin layer. The second molten plastic material is formed into the core layer. Conversely, as shown in FIG. 3, the second molten plastic material is used for the skin layer and the first molten plastic material for the core layer.

[0023] The injection molding processes for the sandwich-shaped article will be described below. First step in operation, when the first molten plastic material is to be used, the first shut-off valve 33 is opened, while the second shut-off valve 32 is closed. By these valves handling, the first molten plastic material 22 delivered from the first injection unit 41 flows down through the first supply passage 31 and the first channel 15, thereby to be injected into the gate 21 leading into the cavity of the mold 20 as shown in FIG. 2(a).

[0024] Second step in operation, when first shut-off valve 33 is closed and the second shut-off valve 34 is opened, the second molten plastic material 23 delivered from the second injection unit 42 flows down through the second supply passage 32 and the second channel 16, thereby to be injected into the cavity of the mold 20 as shown in FIG. 2(b). It these circumstances, the surface of the first molten plastic material 22 injected prior to the second molten plastic material 23 is in direct contact with the interior surface of the mold 20, then the solidification of the surface of the first molten plastic material 22 begins gradually by being cooled. Therefore, the interior of the first molten 22 is filled with the second molten plastic material 23 which is injected subsequently to the solidification so that the second molten plastic material 23 is formed into the core layer, the first molten plastic material 22 into the skin layer.

[0025] When the second molten plastic material 23 injected into the mold 20 reaches the predetermined amount necessary to form the core layer, the second shut-off valve 34 is closed, and the first shut-off valve 33 is opened for the second time to inject only the first molten plastic material 22 into the mold 20, allowing the first molten plastic material 22 to completely encloses the core layer of the second plastic as shown in FIG. 2(c). Thus, the injecting processes for the sandwich-shaped having the skin layer and the core layer is completed.

[0026] On the other hand, FIG. 3 shows the injecting processes for molding sandwich-shaped article having the core layer made of the first molten plastic material 22 and the skin layer made of the second molten plastic material 23. While second molten plastic material 23 is injected into the mold 20, flowing down through the second supply passage 32 and the first channel 16, if the second molten plastic material 23 reaches a predetermined amount, Then the first molten plastic material 22 is injected into the mold 20, flowing down from the first passage 15 while continuing to inject the second molten plastic material 23. It these circumstances, the surface of the second molten plastic material 23 injected prior to the first molten plastic material 22 is in direct contact with the interior surface of the mold 20, then the solidification of the surface of the second molten plastic material 23 begins gradually by being cooled. Therefore, the interior of the second molten 23 is filled with the first molten plastic material 22 being injected subsequently to the solidification so that the first molten plastic material 22 is formed into the core layer. When the first molten plastic material 22 being injected into the mold 20 reaches the predetermined amount necessary to form the core layer, the injection of the first molten plastic material 22 is stopped. When the second shut-off valve 34 is opened, and the first shut-off valve 33 is closed to inject only the second molten plastic material 23 into the mold 20, allowing the second molten plastic material 23 to completely encloses the core layer of the first plastic 22 as shown in FIG. 3(c).

[0027] A material changing method in an injection molding according to the present invention, which is carried out by means of the above described injection nozzle for the sandwich-shaped article, will be described below.

[0028] First Embodiment

[0029] This first embodiment is applicable to injection molding processes for the sandwich-shaped article consisting of the skin layer and the core layer as shown in FIG. 2. and FIG. 3. In this first embodiment, the first injection unit 41 will replace the first molten plastic material 22 which is previously forming the skin layer with a third molten plastic material while continuing injection molding.

[0030] This material changing process includes a injection changing step for reversing the relationship between the skin layer of the first plastic material 22 and core layers of the second molten plastic material 23 , and a material changing step for changing the material which is to be supplied to the first injection unit 41, from the first plastic to the third plastic.

[0031] With respect to the sequence of the injection changing step and material changing step, the material changing step precedes with the injection changing step. And the injection changing step is carried our before plastic material mixed with the first molten plastic material 22 and the third molten plastic material is introduced into the first passage 15.

[0032] Before changing the injection passages ( the second shut-off valve 34 is opened, the first shut off valve 33 is closed), the second molten plastic material 23 injected from the second passage 16 forms the skin layer as shown in FIG. 3(a). After changing the injection passages ( the second shut-off valve 34 is closed, the first shut off valve 33 is opened) as shown in FIG. 3(b), the first molten plastic material 22 mixed with the third molten plastic material is injected from the first passage 15 so as to form the core layer (mixed plastic molding step).

[0033] While the replacement the first molten plastic material 22 by the third molten plastic material is progressing, the mixed plastic of the first molten plastic material 22 with the third molten plastic material is confine into the skin layer of the second molten plastic material 23 to form the core layer without appearing on the surface of the molded article. Therefore, if the sandwich-shaped molded article, as such as the above described mixed plastic is confined in the skin layer, is acceptable for the product, the mixed plastic produced in consequence of the changing material process can be fully utilized without being uselessly wasted.

[0034] While continuing the injection cycle for the sandwich-shaped molded article using the mixed plastic, the mixed plastic injected from the first passage 15 will be completely changed into the third molten plastic material. From that time, by changing the injection sequence, the third molten plastic material may be used for the skin layer, and the second molten resin 23 may be used for the core layer. Alternatively, the injection molding process using the third molten plastic material for the core layer, the second molten plastic material for the skin layer, may be continued without changing the injection sequence.

[0035] In a case where the second molten plastic material 23 for core layer is changed to the fourth molten resin, while the injection molding as shown in FIG. 2 is carried out, the injection sequence changing process is not required, and the plastic to be supplied to the second injection unit 42 may be changed from the second molten plastic material to the fourth molten plastic material.

[0036] Second Embodiment

[0037] While the injection molding process for the sandwich-shaped article has been carried out before and after the plastic changing process in the above described first embodiment, the present invention should not be limited thereto. The injection molding process for the non-sandwich article using only the first or second molten plastic material 22 or 23 may be changed to a injection molding process for non-sandwich article using only the second or third molten plastic material via the above described mixed plastic molding process, or to injection molding for sandwich-shaped article using both of the molten plastic material.

[0038] Second embodiment is directed to a injection molding where the first molten plastic material 22 is changed to the third molten plastic material during injection molding for non-sandwich article independently using the first molten plastic material 22 or the second molten plastic material 23.

[0039] First, the injection molding process using only the first molten plastic material 22 is changed to the injection molding process for the sandwich-shaped article using the first molten plastic material 22 for the core layer and the second molten plastic material 23 for the skin layer as shown as FIG. 3. At this time, the second injection unit 42 is in a state of preparation that the second molten plastic material 23 can be injected into the mold 20.

[0040] Before or after starting the injection molding for the sandwich-shaped article, the first molten plastic material 22 is changed to the third molten plastic material. Until the material changing in the first injection unit 41 is completed, the injection molding for the sandwich-shaped article having the skin layer of the second molten plastic material 23 in which the mixed plastic as the core layer is confined is carried out as described above. When the replacement of the first molten plastic material 22 by the third molten plastic material is completed, the first injection unit 41 completely prepared for injecting the third molten plastic material is suspended, and the first shut-off valve 33 is closed to change the process into the injection molding process for non-sandwich-shaped article using only the second injection unit 42 injecting only second molten plastic material 23.

[0041] When the total of molded articles using only the second molten plastic material 23 and articles using the above described mixed plastic reaches a predetermined quantity of production, the molding process by the second injection unit 42 injecting the second molten plastic material 23 is completed. Then, the injection molding using first injection unit 41 which is capable to inject only the third molten plastic material can be carried out. Thus, the present invention is also applicable for a material changing process in a injection molding using a single plastic.

[0042] In the above described preferred embodiment, the injection or stopping of injection of the first and second molten plastic material 22 and 23 into the metal 20 has been carried out by the first and second shut-off valves 33 and 34 provided in the first and second supply passages 31 and 32. However, the first and second shut-off valves 33, 34 may be replaced with a retraction preventing mechanism for an injection plunger (not shown), i.e., a screw back pressure blocking mechanism in the case of a hydraulic injection unit or a stop locking device for an injection driving motor in the case of an electric injection unit, and the same effect can be obtained. Thus, the injection nozzle for use in the present invention should not be limited to that shown in FIG. 1, and may be a nozzle applicable to the sandwich-shaped molding article of the first and second molten plastic materials 22 and 23, which are plasticized by the first and second injection units 41 and 42, for any one of the skin and core layers.

[0043] As described above, in the material changing method for the injection molding according to the present invention, the injection nozzle for sandwiched-shape article enable the mixed plastic, which is produced during the material changing process, to be confined to the core layer, and prevent from appearing on the surface of the molded article. Therefore, it is possible to fully utilize the mixed plastic which has been conventionally wasted, and it is possible to continuously produce molded articles while carrying out the material changing process without uselessly causing waste plastics in the material changing process. In addition, the sandwich-shaped molded article according to the present invention can be hardly functionally damaged by using mixed plastic produced in the material changing process.

[0044] While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. 

What is claimed is:
 1. A material changing process for an injection molding, said method comprising the steps of; providing a first injection unit for plasticizing and injecting a first molten material and a second injection unit for plasticizing and injecting a second molten material and an injection nozzle connected to the first and second injection unit so that either of the first molten material or the second molten material selectively is injected into a mold; changing the first molten material being plasticized in the first injection unit to a third molten material; injecting the second molten material in the second injection unit into the mold to form a skin layer of a sandwich-shaped article; injecting a mixed molten material being mixed the first molten material with the third molten material in the first injection unit into the mold to form a core layer of the sandwich-shaped article; and repeating a injection molding cycle for molding the sandwich-shaped article until the mixed molten material in the first injection unit is completely replaced with the third molten material.
 2. A process according to claim 1, wherein said material changing process is included in a injection molding process for the sandwich-shaped molded article.
 3. A process according to claim 1, wherein said material changing process is included in a injection molding process for a molded article made of single plastic material.
 4. A process according to claim 2 or 3, wherein said material changing process is introduced between the injection molding process for the sandwich-shaped molded article and the injection molding process for the molded article made of single plastic material.
 5. A sandwich shaped-molded article formed by a injection molding, said article comprising; a skin layer; and a core layer confined in the skin layer, wherein said core layer is made of mixed plastic material generated during a material changing process in a injection unit. 